Electron beam focusing system including a magnetically permeable shield enclosing theelectron gun permanent magnet



Nov. 14, 1967 w. HERSHYN ELECTRON BEAM FOCUSING SYSTEM INCLUDING A MAGNETICALLY PERMEABLE SHIELD ENCLOSING Filed April 24. 1964 THE ELECTRON GUN PERMANENT MAGNET 2 Sheets-Sheet 1 PM+I 'PPM m I q W T ,o.4- 5 l o2- q i w i E O Axis -o.2 P 3 0: PRIOR ART INVENTQR. U L L WILLIAM HERSHYN BY -O 8 U x ATTORNEY Nov. 14, 1967 w. HERSHYN ELECTRON BEAM FOCUSING SYSTEM INCLUDING A MAGNETICALLY PERMEABLE SHIELD ENCLOSING THE ELECTRON GUN PERMANENT MAGNET 2 Sheets-Sheet 2 Filed April 24. 1964 2mm w w y 2mm 2 z w m i and W F w z/ zzwwzwcvmm l mm\[\ .5 NW w mvm =wwlk mm wm rn Z l\ mm hm\ml\ \V mm mm m j 3 j 3 In? IWOIH wo w C L I i Q a IQO INVENTOR. WILLIAM HERSHYN ATT ORNEY United States Patent Oflice 3,353,056 Patented Nov. 14, 1 967 ELECTRON BEAM FGCUSING SYSTEM INCLUD- ING A MAGNETICALLY PERMEABLE SHIELD ENCLOSING THE ELECTRON GUN PERMA- NENT MAGNET William Hershyn, San Francisco, Calif, assignor to General Electric Company, a corporation of New York Filed Apr. 24, 1964, Ser. No. 362,359 3 Claims. (Cl. SIS-3.5

ABSTRACT OF THE DISCLOSURE An electron beam focusing structure for a traveling wave tube system which includes magnetic flux deflection means positioned so as to deflect a portion of the magnetic flux and thereby eliminate undesirable distortion of the axial magnetic field.

This invention relates to a magnet system of relatively light weight for focusing the electron beam of a relatively low-noise traveling wave tube or the like.

In a typical traveling wave tube a beam of electrons is projected by an electron gun closely past an interaction circuit along which an electromagnetic wave is propagated, the beam eventually being intercepted by a collector electrode. To prevent dispersion of the electrons of the beam, to prevent the electrons from striking the interaction circuit and to confine the electrons to regions of high signal fields, an axial or longitudinal magnetic field is ordinarily provided to focus the beam.

It is now well-known that the weight of a beam focusing magnet system can be substantially reduced by the use of a periodic magnetic field as compared to a uniform field. Such periodic magnetic fields have been discussed, for example, by J. T. Mendel et al. in an article entitled Electron Beam Focusing With Periodic Permanent Magnet Fields, Proceedings of I.R.E., volume 42, pp. 800 810, May 1954. An advantage attendant the reduction in weight of the magnet system is the reduction of fringing fields which allows close spacing between tubes or between a tube assembly and other magnetic material without the need of bulky and heavy magnetic shielding.

In typical periodic focusing arrangements a relatively short magnetic period is provided. A short magneti period minimizes the ripple in the electron beam diameter and allows minimization of size and weight of the focusing structure. However, short magnetic period structures give rise to non-laminar electron flow with consequent noise in a traveling Wave tube. Thus it has been found that the noise figure of traveling wave tubes focused by short period magnetic fields is in the order of 3 db higher than tubes focused with uniform fields.

To achieve a low noise figure in a traveling wave tube amplifier it is found particularly necessary to maintain laminar flow of the electron beam in the region of the electron gun and preferably for a distance along the axis required for the amplification of the electromagnetic wave to about db with respect to the input electromagnetic wave.

To this end it has been proposed to employ a gun magnet to provide a uniform gun magnetic field over the gun end of the tube with a periodic magnetic field of relatively short period over the remainder of the tube. In this way the disturbance of the beam is greatly reduced along the critical first portion of the beam path.

It is desirable to employ a permanent magnet to provide the gun magnetic field rather than a solenoid which would require a bulky and expensive power supply. However, unlike a solenoid, a permanent magnet provides not only a desired major lobe of magnetic flux but also an undesirable minor lobe of opposite magnetic polarity at each end thereof.

In particular the minor lobe (toward the collector end of the tube) of the gun permanent magnet causes an undesirable distortion of the axial magnetic field in the transition from the gun field to the following periodic field.

Prior attempts to alleviate this difficulty include the use of magnets of special size and shape formed of special high magnetic strength materials at the beginning of the periodic magnet assembly to buck out or cancel this undesired minor lobe of the gun magnet. Such an approach has had varying degrees of success and is always expensive. Furthermore in such an arrangement the gun magnet and the periodic magnet structure with its special magnets are highly dependent. In other words, a change in the gun magnet necessitates redesign of the special magnets.

It is desirable to overcome the deleterious elfects of this minor lobe of the gun magnet without the necessity of expensive special compensating magnets or the like.

It is further desirable to substantially eliminate the undesirable minor lobe of the gun magnet with structure substantially independent of the periodic magnet assem bly.

It is therefore a primary object of this invention to deflect the undesirable minor lobe of the gun magnet from the path of the electron beam.

It is a further object of the invention to provide a substantially undistorted transition between a relatively long gun magnetic field and a periodically reversing magnetic field of relatively short period without the use of special compensating magnets.

These and other objects are achieved according to the present invention by providing a magnetic flux deflection structure for deflecting the undesirable magnetic flux of the minor lobe of the gun magnet from the axis of the magnet system. More specifically, the gun magnet is enclosed in a spaced magnetically permeable shield structure. The collector end of the gun magnet is fitted with a magnetically permeable pole piece which cooperates with the shield structure to provide a low reluctance path for the magnetic flux of the undesired minor lobe of the gun magnet whereby the minor lobe flux is deflected from the path of the beam.

The invention is described more specifically with reference to the accompanying drawings in which:

FIGURE 1 is a longitudinal section view of a magnet system according to the invention with a traveling wave tube structure shown schematically;

FIGURE 2 illustrates the axial field of the gun magnet with and without the minor lobe deflection structure of the invention;

FIGURE 3 illustrates the axial magnetic field and the distortion thereof by the minor lobe of the gun magnet when the deflection structure of the invention is absent;

FIGURE 4 illustrates the axial magnetic field and the relatively undistorted transition from the gun magnet field to the periodic field when the minor lobe deflection structure of the invention is present;

FIGURE 5 is a partial longitudinal section view illustrating a first alternate form of deflection structure; and

FIGURE 6 is a partial longitudinal section view a second alternate form of deflection structure.

Illustrating in FIG. 1, in longitudinal section view, is a first embodiment of the magnet system according to the invention as adapted to focus the electron beam of a traveling wave tube.

A traveling wave tube of well-known type, is illustrated schematically in FIG. 1, The elements of such a tube comprise an electron gun, including a cathode 10 and a plurality of focusing and accelerating electrodes 11 and 12 03 for projecting a beam of electrons through a slow wave circuit, illustrated as a helix 13, to a collector electrode 14. These elements of the traveling wave tube are enclosed in an evacuated envelope (not shown) and appropriate operating voltages are applied to a plurality of terminals 15, 16, 17 and 18 connected to respective tube elements. As illustrated in FIG. 1, a terminal 19 is provided to receive and apply input electromagnetic energy to the helix 13 for propagation thereby, and a terminal 20 is provided to receive electromagnetic energy from the helix 13. However, many suitable input and output coupling arrangements are known in the art.

The magnet system for focusing the electron beam includes a cylindrical or annular gun permanent magnet 21 and a plurality of shorter annular permanent magnets 22(l)22(m), the later comprising a periodic focusing structure.

As explained hereinbefore, the comparatively long gun magnet provides a uniform longitudinal or axial magnetic field over the gun end of the tube and along the initial portion of the path of the electron beam whereby lownoise operation is achieved.

The periodic magnet assembly, comprising magnets 2-2(l)22(m) with a plurality of pole pieces 23(l)23(n), provides a light weight focusing structure for focusing the electron beam along the remainder of its path.

This magnetic system thus provides the advantage of low noise operation while retaining to a significant extent the light weight and small size of a periodic focusing structure.

A problem however with this type of focusing structure is to achieve an undistorted transition from the uniform axial field of the gun magnet to the alternating magnetic field of the periodic magnet assembly, this problem arising, as discussed hereinbefore, because of the minor lobe of the gun permanent magnet toward the collector end of the tube.

FIG. 2 is a curve illustrating the polarity and relative axial magnetic field intensity of gun magnet 21. The dashed curve illustrates the axial magnetic field intensity of gun magnet 21 when standing alone and fitted with a conventional pole piece at each end similar to a pole piece 24 at the left end of gun magnet 24 as illustrated in FIG. 1. (The solid curve illustrates the axial magnetic field when the minor lobe deflection structure of the invention is provided as described hereinafter.)

Thus referring to the dashed curve of FIG. 2, it is noted that the gun magnet 21 provides a desired major lobe 31, a minor lobe 32 at the lefthand end (which causes no difliculty in the present arrangement) and an undesirable minor lobe 33 at the righthand end. It is thus the minor lobe 33 which it is the purpose of the present invention to reduce or eliminate.

The minor lobe 33 is reduced and substantially eliminated according to the present invention by providing a special annular deflection pole piece 25 (FIG. 1), formed of magnetically permeable material and positioned at the righthand or collector end of the gun magnet 21.

The pole piece 25 is formed with a flange portion 26 and a radially extending portion 27, this latter portion forming a gap 28 with a shield structure 29 (formed of magnetically permeable material) which surrounds the magnet systern.

Thus the pole piece 25, with its radially extending portion 27, cooperates with the shield structure 29 to form a relatively low reluctance path through gap 28 whereby the magnet flux of minor lobe 33 is conducted around the outside of gun magnet 21 and towards the gun end of the focusing system. The magnetic flux of minor lobe 33 is thereby deflected from the axial path of the electron beam. This structure is elfective to substantially eliminate the axial component of the minor lobe 33. As illustrated in FIG. 2, the axial component of the minor lobe 33 is thereby reduced to a residual minor lobe 33'.

Also, as illustrated in FIG. 2, this deflection structure causes some shunting, and therefore a small reduction in the intensity. of the major lobe 31. The radial length of the gap 28 can be adjusted to optimize the reduction of the undesired minor lobe 33 with respect to the shunting effect on the major lobe 31. the length of the gap 28 being generally significantly less than the spacing of the remainder of the magnet structure from the magnetic shield structure 29 or even zero.

FIGS. 3 and 4 illustrate the beneficial affects of the minor lobe deflection structure of the invention. FIG. 3 illustrates the relative axial field intensity in the absence of the deflection structure of the invention, the portion designated PM being the gun magnet field and the portion designated PPM being the periodic magnet field. It is noted that Without the deflection structure of the invention the minor lobe 33 of the gun magnet 21 distorts the initial portion or the periodic magnet field, that is, the first part or the periodic field is shifted with respect to the axis which thus results in undesirable perturbation of the electron beam.

FIG. 4 illustrates the axial field intensity resulting from the use of the minor lobe deflection structure of the invention and the consequent absence of distortion in the transition form the gun magnet field PM to the periodic magnet field PPM.

'rlG. 5 is a partial longitudinal section view illustrating a second embodiment or alternate form of minor lobe deflection structure. Parts similar to those of FIG. 1 are similarly numbered. In this embodiment a deflection pole piece 25 is formed with a laterally extending portion 27 to form a relatively low reluctance path across a gap 28 to the shield structure 29 for deflecting the magnetic flux of the undesired minor lobe.

A further embodiment or alternate form is illustrated in FIG. 6. In this embodiment the shield structure 29 is provided with an inwardly extending annular portion 35 having a lateral extension 36 which forms a relatively low reluctance path across a gap 28" to a pole piece 25" and the rightmost portion of gun magnet 21 for deflecting the magnetic flux of the minor lobe 33 (FIG. 2) of the gun magnet.

Thus what has been described is an electron beam focusing structure which provides a uniform magnetic field over the gun end of the tube to achieve low noise operation while employing a light weight periodic structure to focus the beam along the remainder of its path and in which an undistorted transition from the gun field to the periodic field is achieved by a relatively simple, inexpensive and readily modifiable minor lobe deflection structure without the use of special and expensive compensating magnets.

While the principles of the invention have been made c.ear in the illustrative embodiments, there will be obvious to those skilled in the art, many modifications in structure. arrangement, proportions, the elements, materials, and components, used in the practice of the invention, and otherwise, which are adapted for specific environments and operating requirements, without departing from these principles. The appended claims are therefore intended to cover and embrace any such modifications within the limits only of the true spirit and scope of the invention.

What is claimed is:

i. In a traveling wave tube system having an electron gun for pro ecting a stream of electrons along a predetermined path and a collector electrode at the end of said path, a beam focusing structure for preventing dispersion of said beam comprising:

.1 gun permanent magnet structure providing a first .ixially magnetic field having a major lobe extending the major length of the electron gun and a minor lobe adjacent the gun and for focusing said stream iiong a first portion of said path;

a periodic magnet structure providing an alternating magnetic field for focusing said beam along the remainder of said path;

a magnetic flux deflection means positioned adjacent the collector electrode end of said gun magnetic structure for deflecting magnetic flux of the minor lobe of said gun magnetic structure from the path of said beam, said magnetic flux deflection means including a magnetically permeable shield structure positioned in spaced relation and substantially enclosing the electron gun permanent magnet; and

means forming a relatively low reluctance path be tween said shield structure and said collector electrode end of said gun magnet structure so as to shunt the magnetic flux of said minor lobe around the outside of said gun magnet and towards the gun end of the focusing system.

2. In a traveling wave system as recited in claim 1 wherein the gun permanent magnet structure further includes first and second apertured spaced pole pieces positioned coaxially about said electron gun and at either end of the permanent magnet structure; the pole piece adjacent the collector electrode end of said permanent magnet means having a radially extending portion forming a relatively low reluctance magnetic flux path with said shield structure for deflecting the minor lobe flux.

3. In a traveling Wave system as recited in claim 1 wherein the gun permanent magnet structure further includes first and second apertured spaced pole pieces positioned coaxially about said electron gun and at either end of the permanent magnet structure; the pole piece adjacent the collector electrode end of said permanent magnet means having a coaxially extending portion forming a relatively low reluctance magnetic flux path with said shield structure for deflecting the minor lobe flux.

References Cited UNITED STATES PATENTS 2,965,782 12/1960 Weaver 313-84 3,243,639 3/1966 Glass 3153.5 3,252,033 5/1966 Orui 313-84 3,259,788 7/1966 Orui 315-35 ELI LIEBERMAN, Primary Examiner.

HERMAN KARL SAALBACH, Examiner.

P. L. GENSLER, Assistant Examiner. 

1. IN A TRAVELING WAVE TUBE SYSTEM HAVING AN ELECTRON GUN FOR PROJECTING A STREAM OF ELECTRONS ALONG A PREDETERMINED PATH AND A COLLECTOR ELECTRODE AT THE END OF SAID PATH, A BEAM FOCUSING STRUCTURE FOR PREVENTING DISPERSION OF SAID BEAM COMPRISING: A GUN PERMANENT MAGNET STRUCTURE PROVIDING A FIRST AXIALLY MAGNETIC FIELD HAVING A MAJOR LOBE EXTENDING THE MAJOR LENGTH OF THE ELECTRON GUN AND A MINOR LOBE ADJACENT THE GUN AND FOR FOCUSING SAID STREAM ALONG A FIRST PORTION OF SAID PATH; A PERIODIC MAGNET STRUCTURE PROVIDING AN ALTERNATING MAGNETIC FIELD FOR FOCUSING SAID BEAM ALONG THE REMAINDER OF SAID PATH; A MAGNETIC FLUX DEFLECTION MEANS POSITIONED ADJACENT THE COLLECTOR ELECTRODE END OF SAID GUN MAGNETIC STRUCTURE FOR DEFLECTING MAGNETIC FLUX OF THE MINOR LOBE OF SAID GUN MAGNETIC STRUCTURE FROM THE PATH OF SAID BEAM, SAID MAGNETIC FLUX DEFLECTION MEANS INCLUDING A MAGNETICALLY PERMEABLE SHIELD STRUCTURE POSITIONED IN SPACED RELATION AND SUBSTANTIALLY ENCLOSING THE ELECTRON GUN PERMANENT MAGNET; AND MEANS FORMING A RELATIVELY LOW RELUCTANCE PATH BETWEEN SAID SHIELD STRUCTURE AND SAID COLLECTOR ELECTRODE END OF SAID GUN MAGNET STRUCTURE SO AS TO SHUNT THE MAGNETIC FLUX OF SAID MINOR LOBE AROUND THE OUTSIDE OF SAID GUN MAGNET AND TOWARDS THE GUN END OF THE FICUSING SYSTEM. 